1. Field of the Invention
This invention relates to a device for encoding an image information signal such as a TV signal and more particularly to an encoder which efficiently encodes an image signal through advantageous utilization of the characteristics of the signal.
2. Description of the Related Art
Apparatuses embodying known predictive encoding methods include, for example, a digital VTR, etc. In order to record on a recording medium, such as a magnetic tape, an image signal carrying a great amount of information, the apparatus of this kind has been arranged to accomplish recording by compressing an image data transmission band utilizing the correlativity of the image information.
It is difficult for an ordinary magnetic recording apparatus to record and reproduce a signal component of extremely low frequency and a DC component. The following description shows this in detail on the basis of the recording and reproducing principle of a digital VTR:
Recording and reproduction on and from a magnetic tape are performed via several magnetic heads mounted on a rotary cylinder. However, the magnetic head is generally arranged to perform recording or reproduction by converting the variations of a magnetic flux which take place with time (differential values) into voltages or by reversely carrying out the conversion. Therefore, it is difficult to reproduce DC and low frequency components of the signal. Besides, since the magnetic head incessantly revolves at a high speed, recording and reproduced signals are supplied and received to and from the magnetic head via a rotary transmitter or the like which is mounted on the above stated rotary cylinder. The rotary transmitter is, therefore, also hardly capable of transmitting the DC and low frequency components like the magnetic head. It is thus impossible to have the DC component of the signal transmitted.
To solve this problem, it is practiced not to straightly record band-compressed image data but to suppress the DC component thereof by scrambling it using a pseudo-random pattern before recording or reproduction. In this instance, however, the scrambled image data still contains the DC component in a small amount. Therefore, such a transmission system that consists of the magnetic head and the rotary transmitter and is incapable of transmitting the DC component inevitably gives many detection error in reproducing a DC component or a low frequency component. The increased rate of errors then results in deterioration of picture quality.
Further, various DC-free record modulating methods (such as an 8-10 block encoding method, an interleave NRZI method, etc.) of recording after modulation have been known. However, in the case of conversion without any DC component, like the 8-10 block encoding method, the rate of transmitting bits increases due to an increased degree of redundancy. Therefore, the methods of this kind hardly permit high density recording. In addition to this disadvantage, such modulating methods necessitates a complex process, which results in an increased amount of hardware.
To solve that problem, there has been proposed a predictive encoder as disclosed in U.S. Pat. Application Ser. No. 890,831, filed on July 25, 1986 and assigned to the assignee of the present invention. This encoder is arranged to be capable of suppressing the whole DSV (digital sum value) of the modulated signal by allotting a CDS (code-word digital sum) of a smaller value to a representative difference value which has a higher degree of appearing frequency. The details of this predictive encoder is as described below:
FIG. 1 of the accompanying drawings is a circuit diagram showing the above stated predictive encoder. Referring to FIG. 1, a subtracter 1 is arranged to subtract a predicted value signal P from an incoming image signal Di and to produce a prediction error signal E. A quantizer 2 is arranged to receive the prediction error signal E and to produce a data signal Do which consists of, for example, four bits as will be further described later. A representative value setter 3 is arranged to have a characteristic reverse to that of the quantizer 2. An adder 4 is arranged to add the signal output of a predictor 5 to a representative value signal R and to have the output thereof fed back to the input side of the predictor 5. The representative value setter 3, the adder 4 and the predictor 5 jointly form a local compositor 6 which is arranged to produce the prediction value signal P.
The input-output characteristic of the quantizer 2 is as shown in Table 1 below. Table 1 shows the level of the prediction error signal E in relation to the bit arrangement of the output data signal Do.
TABLE 1 ______________________________________ Prediction error E Output bit pattern Do CDS ______________________________________ 7 1111 +4 6 0010 -2 5 0111 +2 4 0001 -2 3 1101 +2 2 0011 0 1 0101 0 0 0110 0 -1 1010 0 -2 1100 0 -3 1011 +2 -4 1000 -2 -5 1110 +2 -6 0100 -2 -7 0000 -4 ______________________________________
In Table 1 above, each of values CDS indicates the total sum of bits within a single code obtained with the level "1" of the bit pattern at each bit in the output data signal Do assumed to be "+1" and the level "0" to be "-". The value of the CDS becomes zero when the sum of the value "1" is equal to that of the value "0".
The prediction error signal E is known to have a statistical nature that it has a large occurrence frequency distribution around "0" according to the correlativity of image information as shown in FIG. 2. In view of this, the predictive encoder is arranged to make code allotment to a range within which the value of the prediction error signal E is small in such a way as to make the absolute value of CDS small. Further, since the values of the prediction error signal E are symmetrically distributed around "0", the bit patterns of prediction error signals which are equal to each other in the absolute value are inversely allotted or arranged in the output data signal Do as shown in Table 1. The details of the inverse bit allotment of the bit patterns are as described below:
For example, while the output bit pattern is "1101" when the prediction error is "+3", the higher and lower bits are inversely allotted and to obtain a bit pattern of "1011" when the error is "-3". The bit pattern is "0010" for prediction error of "+6" and "0100" for an error of "-6". In the case of this specific example, however, bit pattern allotment is "1111" and "0000" for the maximum prediction error values of "+7" and "-7" respectively. Further, in the case of zero prediction error, the allotted bit pattern "0110" may be replaced with "1001".
As apparent from the above description, bit patterns of smaller absolute values of CDS are allotted to more frequently appearing values of the prediction error signal E around E=0. This arrangement enables the output data signal Do to have a less DC component.
FIG. 3 shows by way of example another known predictive encoder. Referring to FIG. 3, a subtracter 7 is arranged to subtract a prediction value signal P from an incoming image signal Di and to produce a prediction error signal E. A reference numeral 8 denotes a change-over switch. Numerals 9A and 9B denote first and second quantizers (Q1 and Q2). The first and second quantizers 9A and 9B (or Q1 and Q2) are arranged to give quantizing bit patterns as shown in Table 2 below:
TABLE 2 ______________________________________ Prediction error E Q1 output Q2 output CDS ______________________________________ 7 1111 0000 +4/-4 6 0010 0010 -2 5 0111 0111 +2 4 0001 0001 -2 3 1101 1101 +2 2 0011 0011 0 1 0101 0101 0 0 0110 0110 0 -1 1010 1010 0 -2 1100 1100 0 -3 1011 1011 +2 -4 1000 1000 -2 -5 1110 1110 +2 -6 0100 0100 -2 -7 1001 1001 0 ______________________________________
Referring further to FIG. 3, an up-down counter 10 is arranged to up count the output data signal Do when the signal Do is at "1" and to down count it when it is at "0". A reference numeral 11 denotes a change-over switch. First and second representative value setters 12A and 12B (or R1 and R2) are arranged to have characteristics which are reverse to those of the first and second quantizers 9A and 9B respectively. A numeral 13 denotes an adder and a numeral 14 a predictor.
As apparent from Table 2 above, the first and second quantizers 9A and 9B differ from each other in that: They are arranged to produce the output data signal Do in different bit patterns of "1111" and "0000" when the prediction error is "7". As for all other prediction error values "+6" to "-7", the two quantizers produce exactly the same bit pattern. The change-over switches 8 and 11 are arranged to have their connecting positions on their sides A to select the first quantizer 9A and the first representative value setter 12A when sign bit information from the up-down counter 10 indicates the negative sign and on their sides B to select the second quantizer 9B and the second representative value setter 12B when it indicates the positive sign.
This encoder operates as follows: For the sake of illustration, the connecting positions of change-over switches 8 and 11 are assumed to be on their sides A. As shown by the distribution curve of FIG. 2, it is likely that the prediction error signals E are symmetrically distributed in both the positive and negative directions. When the prediction error signal E which is of such distribution is supplied to the first quantizer 9A (Q1), the probability of consecutively having "1" in the output bit pattern becomes larger than that of consecutively having "0". Therefore, the up-down counter 10 more often up counts. As a result, the sign bit of the output comes to indicate the positive sign.
When the sign bit of the up-down counter 10 comes to show the positive sign, the second quantizer 9B (or Q2) and the second representative value setter 12B (or R2) are selected by the change-over switches 8 and 11. The second quantizer 9B (or Q2) then performs a quantizing action. By this, the output bit pattern shifts to a pattern in which the probability of consecutively having "1" exceeds the probability of consecutively having "0". The up-down counter 10 then comes to down count. The encoder thus comes to perform a feedback action. The feedback action makes the probability of having "0" in the output data signal Do equal to that of having "1", so that a code train which is free from a DC component can be obtained.
The predictive encoders of the prior art which are arranged as described above is capable of forming a coded signal by suppressing the DSV on the basis of the appearing frequency of the prediction errors. However, in handling an image signal of a high sampling rate such as a high definition TV signal, the DC suppression and the encoding efficiency must be furthered for overall compression of the transmission band.